The present invention relates in general to die-casting methods and apparata. More particularly, the present invention relates to a method and apparatus for reducing the amount of trapped gas in die castings, namely aluminum and zinc.
Casting porosity due to shrinkage or gas evolution is a factor of concern with many if not most all casting procedures. One such procedure where trapped gas is of concern is in die-casting work, or as it is sometimes called, pressure die casting.
"In this process molten metal in a viscous condition is forced under high pressure into a steel die, which usually has several cavities connected by channels through which the metal flows almost instantaneously. Because of the speed with which the metal fills the die cavities, the die designer must provide adequate vents to permit the rapid escape of air from the cavities, particularly from pockets and corners; otherwise the casting will be porous because of trapped air. The metal must be viscous rather than fluid or the metal also will squirt through the vents, and yet this very viscosity makes the trapping of air more likely. Consequently, only light sections of the casting are likely to be perfectly sound. Brass die castings having relatively heavy sections (more than one-fourth inch thick for example) will commonly be acceptable if they are used for structural purposes only, but are likely to give trouble if there is any machining to be done in the heavy sections." Kent's Mechanical Engineers Handbook, 12th Ed., Wiley Handbook Series, pp. 19-33.
When describing the occurrence of trapped gas in die castings, various terms of art frequently appear and an understanding of these terms is believed helpful to a full understanding of the present invention. The term "gas porosity" refers to the dispersion of fine cavities in the cast metal which result from the fact that some gas is liberated during solidification of the once-molten or viscous metal. Other terms used to describe this type of an effect are microporosity and pinhole porosity. Microporosity is the term that refers to extremely fine porosity caused in castings by shrinkage or gas evolution and is apparent on radiographic films as mottling. The allowable microporosity is covered by ASTM Standard E-192. The term "pinhole" porosity refers to the presence of very small holes scattered through a casting, possibly caused by microshrinkage or gas evolution during solidification. Allowable pinhole porosity is also covered by ASTM Standard E-192.
As indicated by these various terms of art, porosity in castings occurs not only due to shrinkage porosity but also because of trapped gas. The presence of trapped gas results by the atmosphere in the volume of the part cavity being replaced with molten metal. In addition, there is the volume of air that gets pushed in from the shot tube when the cold chamber casting method is used. This volume of air plus the mold-release vapors must be vented out of the cavity if a sound casting is made; however, this has never been done perfectly. There has been a myriad of venting ideas and many attempts to evacuate by reducing the pressure within the part cavity by some means of vacuum pumping, the best of which results in little better than one-half atmosphere reduction. Approaches similar to the evacuation method are also found in certain prior art references. A sampling of those references are listed below, and while none are believed to teach nor anticipate the present invention, they are to some degree instructive as to the types of approaches which have been considered or tried by others.
______________________________________ Patent No. Patentee ______________________________________ 2,958,104 Ohse 3,349,833 Hodler 3,382,910 Radtke 3,752,213 Miki 3,999,593 Kaiser ______________________________________
Ohse discloses a vacuum die-casting process and apparatus which involves the feeding of molten material into a die cavity which is positioned between relatively movable die members and which is communicating with a substantially horizontally extending feed chamber having a volumetric capacity which is greater than that of the die cavity. The process involves the introduction of a charge of molten material through a feed port into a feed chamber wherein the amount of the charge is such that a free space is left above the charge in the portion of the feed chamber extending from the feed port toward the die cavity. As the molten metal is moved toward the die cavity and with the apparatus sealed, the die cavity and the communicating free space in the feed chamber is subjected to a vacuum in order to remove from the die cavity and the communicating space any air or gases.
Hodler discloses a pressure-molding venting method for use with die-casting work wherein an air vent in the die-casting machine is provided with a spring-loaded normally closed valve which may be held open by a trapped body of pressure fluid from the die-closing mechanism. The fluid is released by a pilot valve which responds to the sudden pressure rise in the hydraulic fluid injection mechanism which occurs when the injected liquid metal enters the vent from the wider die cavity.
Radtke discloses a method of pressure die-casting which is intended to be applicable to die-casting machines in which relatively movable die members are closed to form a die cavity which is a negative impression of the article to be produced. The die cavity communicates with a reservoir of molten metal and during the operation of the machine, the die members are brought together in order to form the die cavity and the molten metal is introduced. While this is generally the description of any die-casting machine and its method of use, this particular invention focuses on the problem of porosity in die casting and a possible solution to that problem. The approach to that solution in this particular reference is to purge air from within the die cavity by flushing the cavity with a reactive gas and then sebsequently inject molten metal into the reactive gas-filled cavity whereby the reactive gas combines with the molten metal thus reducing the tendency for voids to form in the casting which is produced.
Miki discloses a die-casting method and apparatus which employs an oxygen flush in order to reduce trapped gas porosity in the resulting die casting. The approach followed is to attempt to concentrate shrinkage pores in the unimportant regions of the product by positively retarding the solidification of the cast metal in those regions so that the strength defects may be reduced in essential or critical regions of the resultant product.
Kaiser discloses a method and apparatus for pore-free die casting wherein a gas which is designed to react with the cast material flushes air from the various openings and spaces within the mechanism as well as from the die cavity. The reactive gas replaces the air in these locations so that the air is not forced into the die cavity by the material when it fills the die cavity. Due to the fact that air in the die cavity would form pores in the cast material, the intent by this particular reference is to flush or remove the air by the introduction of the reactive gas.
None of the foregoing concepts, methods or devices dsiclosed by the listed references offer any noticeable improvement to the general approach of vacuum pumping which, as mentioned, results in little better than a one-half atmosphere reduction. However, by the present invention, a means of achieving a much greater efficiency is provided. Calculations relating to the method and apparatus of the present invention have indicated the ability to achieve approximately 80-90% efficiency in reducing the amount of gas weight resident in the shot tube and part cavity.